NUCLEAR PHYSICS & RADIOACTIVITY PHYSICS - UNIT ONE

ATOMIC STRUCTURE Atoms consist of: Nucleus - The solid centre of the atom, containing protons and neutrons. - Most of the mass of an atom is in the nucleus. - The collective name for all particles found in the nucleus is nucleons. Protons are positively charged, with relative charge +1. Neutrons are neutral and have no charge.

ATOMIC STRUCTURE Atoms consist of: Electrons - Negatively charged particles, with relative charge of -1 - Found around the nucleus of an atom in ‘shells’ or ‘clouds’. - These shells are arranged in energy levels – so the innermost shell has the lowest energy level and is filled first.

ATOMIC STRUCTURE The amount of protons, neutrons, electrons in an atom are represented as: Mass Number – The total number of nucleons in the nucleus (Neutrons + Protons) Atomic Number – The total number of protons in the nucleus Chemical Symbol for the element The number of electrons in an atom is equal to the number of protons

ATOMIC STRUCTURE Mass Number – The total number of nucleons in the nucleus (Neutrons + Protons) Atomic Number – The total number of protons in the nucleus Chemical Symbol for the element

HISTORY OF THE ATOM - VIDEO

ATOMIC STRUCTURE – VIDEO CLIP

ISOTOPES An atoms name is based on how many protons it has. Not all atoms of the same name have the same number of neutrons. We call atoms with variations in number of neutrons, isotopes of that element. The isotopes therefore have the same Atomic Number (number of protons), but different mass numbers.

ATOMIC STRUCTURE - QUESTIONS

WHY DOES NUCLEAR RADIATION HAPPEN?? Not all atomic nuclei (plural of nucleus) are particularly stable. In some instances, one isotope of an atom may be stable while another is not. This is why nuclear radiation is released! It is the unstable atoms way of becoming more stable. These unstable isotopes are called radioisotopes. Radioisotopes and very common and many of these occur naturally. Many more are produced artificially.

TYPES OF NUCLEAR RADIATION Unstable isotopes can emit various types of radiation while striving to become stable. The radiation that is emitted is ionising radiation, which has the ability to knock electrons from atoms they come into contact with. The 3 naturally occurring forms of nuclear radiation are: Alpha Radiation α Beta Radiation β Gamma Radiation γ

ALPHA DECAY An unstable nucleus ejects a relatively large particle known as an α particle. The α particle consists of 2 protons and 2 neutrons (a Helium nucleus). The remaining daughter nucleus is more stable and is now a different element with 2 less protons and 2 less neutrons.

ALPHA DECAY Or can be written as… This is now a different element

ALPHA DECAY – MORE FUN FACTS! The daughter atom now has 2 electrons too many, however these are lost soon after the decay leaving the atom electrically neutral. In addition, the alpha particle slows down and loses kinetic energy. At low energies the alpha particle will acquire 2 electrons and become a Helium atom. Alpha particles travel at 15,000 km/s This relatively slow velocity means they are very likely to interact with other atoms lose their energy – so they can be stopped within a few cm’s of air. Most of the Helium on earth is a result of alpha decay. Underground deposits of Uranium and Thorium are disrupted during natural gas production, releasing Helium into the atmosphere.

BETA DECAY Two types- β + (produced under laboratory conditions only ) - β - (occurs naturally) β - particle is a fast moving electron that is ejected from an unstable nucleus. In fact it’s true! The outer shell electrons don’t change, instead other interesting changes take place in the nucleus…… Wait! An electron emitted from the nucleus??? Nuclei doesn’t contain electrons!

BETA DECAY One neutron transforms into a proton and an electron. The proton remains in the nucleus, the electron is emitted and is called the β - particle. The resulting daughter nucleus has the same number of nucleons as the parent, but one less neutron and one more proton. Or can be written as…

BETA DECAY – MORE FUN FACTS! Beta particles are very light compared to alpha particles. The travel at a large range of different speeds – from that of an alpha particle (15,000 km/s) up to 99% of the speed of light.

GAMMA DECAY The nucleus is unstable after α or β decay and may need to release energy. Gamma emission occurs after another form of nuclear decay has taken place, when the nucleus is ‘excited’. A small packet of excess electromagnetic energy called a gamma ray is emitted.

GAMMA DECAY – MORE FUN FACTS! Gamma rays: Have no mass Have no charge Are not deflected by magnetic of electric fields Because it is a photon or packet of electromagnetic energy and has no mass to weigh it down, it travels at the speed of light

PENETRATION POWER The various forms of nuclear radiation are so different, so they of course react differently when coming into contact with matter after being ejected from the nucleus. They can each be absorbed by: α particles - a few centimetres of air - a piece of paper - a layer of dead skin β particles - about 100 centimetres of air - a few centimetres of Aluminium γ rays - barely affected by air - absorbed in many centimetres of Lead

QUESTIONS

IONISING POWER An ion is an atom with an overall positive or negative charge. Positive ions form when electrons are removed from a neutral atom. Negative ions form when electrons are added to a neutral atom. Alpha, Beta & Gamma radiation have the ability to ionise atoms they come into contact with.

IONISING POWER α particles - Are slow-moving particles - They have time to interact with most atoms in their path - When they interact with an atom the α particles positive charge attracts electrons from the atoms - These atoms they interact with are ionised – they are no longer neutral - With each ionisation, the α particle slows down and loses energy - Alpha particles have a poor penetrating power - High ionising power

IONISING POWER β - particles - Are repelled by the electrons in atoms - The repulsion causes the β - particles to bounce between atoms. - Each β collision loses less energy than α collisions - This means that β particles have higher penetrating ability than α particles but less ionising power.

IONISING POWER γ rays - May interact with electrons or nuclei they collide with as they move through a substance. - As γ rays have no charge, collisions are infrequent. - Collisions occur only when a nucleus or electron is directly in the path of the γ ray (unlikely due to the large amount of empty space in an atom). - Very low ionising power. - Very high penetrating power.

QUESTIONS